Help - kids are acting out more at my house lately

The last month my boys (6 and 9) have been way more defiant and moody when they're with me. Talking back, not listening, general attitude stuff. They seem fine at their mom's house according to her. I'm wondering if something's wrong or if this is just a phase? I don't want to overreact but I also don't want to miss something actually going on. Should I talk to their mom about this or handle it on my own?